In the field of digital communications, a need exists to successfully communicate digital data with lower signal-to-noise ratios. At lower signal-to-noise ratios, less power is consumed and/or communications take place over greater distances at given bit error rates. Alternatively, if power levels and communication ranges remain constant, the ability to distinguish signals from noise improves and bit error rates improve.
Coherent demodulation and differentially coherent demodulation, or simply differential demodulation, are conventional techniques used for retrieving data conveyed by a carrier signal. Between the two techniques, coherent demodulation typically achieves a 1-3 dB improvement in performance. Generally speaking, during each symbol coherent demodulation retrieves data from a carrier signal by comparing a phase relationship between the signal's quadrature components with an absolute phase reference. Differential demodulation uses a phase reference established by the phase relationship detected in a previous symbol. Noise influences only the carrier signal's phase relationship and not the absolute phase reference in coherent demodulation. Noise influences both the carrier signal's phase relationship and the phase reference in differential demodulation. This doubling of the noise influence causes differential demodulation to exhibit poorer performance than coherent demodulation.
In spite of coherent demodulation's improved performance, more and more digital communication systems rely upon differential demodulation. Differential demodulation is useful because of difficulties in establishing and maintaining the absolute phase reference required for coherent demodulation. In particular, coherent demodulation requires the communication of many symbols of dummy or preamble data in order to establish the absolute phase reference. During this acquisition period, detected data are typically too error prone to be useful. In burst communication applications, the overhead burden of a sufficient number of acquisition symbols is intolerable.
Moreover, coherent demodulation suffers in robustness when compared to differential demodulation. When a carrier signal experiences a significant phase perturbation during an on-going stream of communication, such as from significant Doppler, multipath or interference, coherent demodulation may operate for a period of time on an invalid estimate of the absolute phase reference. This period may continue for a relatively long time until a more accurate absolute phase reference is established. During this period a significant number of data errors may be produced. On the other hand, when differential demodulation experiences such a perturbation, data errors are produced for only a single or a few symbols.
The advantages of differential demodulation when compared to coherent demodulation have lead to efforts to devise alternate demodulation schemes that exhibit improved performance. For example, maximum likelihood and decision-directed schemes have been devised in an attempt to achieve the performance of coherent demodulation with the rapid acquisition characteristics and robustness of differential demodulation. However, such schemes typically require complex implementations and in some cases result in performance which can be even worse than differential demodulation.